In general, gear mechanisms, such as a trapezoidal thread worm gear mechanism or a rack and pinion gear mechanism, have been used as the mechanism to convert rotary motion of an electric motor to an axial linear motion in an electric actuator. These motion converting mechanisms utilize sliding contact portions and thus power loss. Accordingly, they are obliged to increase the size of the electric motors and power consumption. Accordingly, the ball screw mechanisms have been widely adopted as more efficient actuators.
For example in a relatively small boat having a screw driven by an internal combustion engine, the direction change operation of screw rotation between forward and backward directions is carried out by switching a dog clutch, via a wire connected to a lever operated by an operator, to select a forward or a rearward gear. However, an electric actuator for switching the dog clutch has been developed in recent years for labor saving.
In this case, the electric actuator is required to accurately detect the position of the dog clutch in switching the forward and backward directions of the boat and to perform the switching operation. For example, the rotation angle is transmitted to a potentiometer, via a sensor gear, in accordance with the stroke of the moving shaft to obtain the stroke of the moving shaft by detecting its absolute position. However, this requires an increase in the gear ratio of the sensor gear when the stroke of the moving shaft is increased. Thus, this increases the gear size or the gear stage of the sensor gear and brings an increase in the size of the electric actuator. In addition, it is believed that the accuracy of the rotation angle detected by the potentiometer would be detracted due to backlash or pitch error of the sensor gear. Furthermore, it is believed that high detection accuracy would not be obtained due to the generation of a shift of the zero point when the potentiometer is used in a place such as the inside of the electric actuator having a high environmental temperature, due to heat generation of a motor. On the other hand, it is possible to use the potentiometer under a circumstance of ordinary temperature when the potentiometer is mounted on the outside of a housing of the electric actuator. However, in such a case, since the potentiometer is contaminated by sea water, fuel etc., it is necessary to have a water-tight or oil-tight structure or to have a separate structure to provide electromagnetic shielding, which would increase the manufacturing cost of the electric actuator.
An electric actuator 100 is known that can solve these problems, as shown in FIG. 5. The electric actuator 100 includes a housing 101 and an electric motor 102 mounted within the housing 101.
The housing 101 includes a housing body 101A, a cover member 101B mounted on an end face of the housing body 101A and a plate-like motor bracket 101C. A motor chamber 101a and a screw thread shaft chamber 101b are formed within the housing body 101A. The electric motor 102 is arranged within the motor chamber 101a. The electric motor 102 is secured on the motor bracket 101C. The motor bracket 101C is mounted to sandwich an outer race of a ball bearing 114 against the housing body 101A. The motor bracket 101C blocks both the motor chamber 101a and the screw thread shaft chamber 101b of the housing body 101A.
The electric motor shaft 102a projects from the motor bracket 101C. A first gear 103 is securely press fit onto the end of the shaft 102a incapable of rotation relative to the shaft 102a. A second gear 105, of resin material, is freely rotationally mounted on a long shaft 104 secured to the motor bracket 101C. The second gear 105 meshes with the first gear 103 and a third gear 106. The third gear 106, of resin material, is mounted on the end of the screw thread shaft 107 and is incapable of relative rotation to the shaft 107, via a serration connection. The left-side portion of the screw thread shaft 107 is formed with a male thread groove 107a. The right-side portion is rotationally supported by a ball bearing 114 relative to the housing body 101A.
The screw thread shaft 107 is passed through a cylindrical nut 115. The inner circumference of the nut 115 is formed with a female thread groove 115a opposing the male thread groove 107a. A large number of balls 116 are rollably contained in a helical passage formed by the male and female thread grooves 107a, 115a to form the ball screw mechanism. The nut 115 is held within the screw thread shaft chamber 101b so that it can axially move but cannot rotate relative to the housing body 101A. The ball screw mechanism and a cylindrical moving shaft 117 form a driving mechanism.
The left-side end of the screw thread shaft 107 is inserted into a blind bore 117a formed in the moving shaft 117. The right-side end of the moving shaft 117 is coaxially fit into the nut and secured to it by pins to integrally move with it. The moving shaft 117 is axially movably supported by a bush 118 axially movable relative to the housing body 101A. An aperture 117b, for connecting to a link member (not shown), is formed in an end of the moving shaft 117 projecting from the housing body 101A.
FIG. 6 is a schematic view of the inside structure of the electric motor 102. FIG. 7 is a cross-section view taken along a line VII-VII. As shown in these drawings, an annular magnet MG is secured on a shaft 102a on which a rotor 102d is mounted. The annular magnet MG is separated to half annular portions MGs, MGn arranged at both sides of the rotation shaft 102a. The half annular portion MGs has the S pole on its outer circumference and the half annular portion MGn has the N pole on its outer circumference. First and second sensors SA, SB are mounted on the inner wall of a motor housing 102c shifted 90° to each other around the axis of the shaft 102a. 
The stroke of the moving shaft 117 is determined by the amount of rotation of the rotation shaft 102a of the electric motor 102, the gear ratio of the gears 103, 105, 106 and the lead of the ball screw mechanism. Since the gear ratios and the lead are known values, it is possible to detect the position of a dog clutch (not shown) if the amount of rotation can be exactly measured. Thus, it can be found that the annular magnet MG is rotated in a clockwise direction (CW) when the wave phase of the sensor SB is advanced from that of the sensor SA. On the contrary, it can be found that the annular magnet MG is rotated in the counterclockwise direction (CCW) when the wave phase of the sensor SB is delayed from that of the sensor SA. Accordingly, it is possible that the control apparatus ECU can accurately obtain the stroke and the moving direction of the moving shaft 117 by the output signals of the sensors SA, SB. See, Japanese Laid-open Patent Publication No. 2008-274971.
In the prior art electric actuator 100, the sensors SA, SB output pulse signals, in accordance with the rotation of the electric motor 102, and are arranged within the housing 102c of the electric motor 102. Also, the control apparatus ECU is provided to determine the stroke and the direction of the moving shaft 117 by the outputs from the sensors SA, SB. Thus, it is possible to protect the sensors SA, SB from the external environment and to improve the reliability of the electric actuator as a system.
However, the structure and technology for arranging the sensors SA, SB within the housing 102c of the electric motor 102 requires special knowledge and designing manner. Thus, it is necessary to use an electric motor with a special design. Thus, the versatility of the electric motor is detracted. In addition, this increases the manufacturing cost of the actuator since it is impossible to use a versatile electric motor in various applications requiring different performances.